1. Field of the Invention
This invention relates to a method of geophysical prospecting which improves the accuracy of seismic migration. Specifically, the invention uses a walkaway VSP survey for determination of subsurface velocities and imaging of reflections below salt layers in the earth.
2. Description of the Related Art
In surface seismic exploration, energy imparted into the earth by a seismic source reflects from subsurface geophysical features and is recorded by a multiplicity of receivers. This process is repeated numerous times, using source and receiver configurations which may either form a line (2-D acquisition) or cover an area (3-D acquisition). The data which results is processed to produce an image of the reflector using a procedure known as migration.
Conventional reflection seismology utilizes surface sources and receivers to detect reflections from subsurface impedance contrasts. The obtained image often suffers in spatial accuracy, resolution and coherence due to the long and complicated travel paths between source, reflector, and receiver. Salt layers in the subsurface are particularly problematic. Due to the high compressional wave (P-wave) velocity of salt (4.48 km/s or 14,500 ft/s), there is considerable ray-bending of P-waves at the top and bottom of salt layers due to the large velocity contrast. Typical sedimentary velocities in the Gulf of Mexico may be no more than 3 km/s.
Numerous approaches have been taken to address the problem of sub-salt imaging. These include using low frequencies, use of prestack depth migration, use of converted waves, redatuming to the base salt reflection, and seismic inversion. These have had limited success.
The present invention uses a walkaway Vertical Seismic Profile (VSP) survey to estimate sub-salt velocities by tomographic inversion of reflection travel-times. In a walkaway VSP survey, measurements are made using a plurality of receivers in a borehole responsive to excitation of one or more seismic sources at a plurality of distances from the wellbore. The estimated velocities may then be used for migration of the walkaway VSP data or of surface seismic data. This method is particularly useful in the drilling of offset wells where an initial well that may or may not be productive has been drilled. Using the method of the present invention, it is possible to image the subsurface of the earth away from the initial well.